The present invention relates to drill bits for use in rotary drilling through earth formations.
In a typical rotary drilling operation, a rotary drill bit is rotated while being advanced into a soil or rock formation. The soil or rock is cut by cutting elements on the drill bit, and these cuttings are flushed from the borehole by the circulation of drilling fluid toward the top of the borehole. The drilling fluid is delivered to the drill bit downwardly through a passage in the drill stem and is ejected outwardly through nozzles disposed in the cutting face of the drill bit. The ejected drilling fluid is directed outwardly through the nozzles at high speed to aid in cutting, and to flush the cuttings and cool the cutter elements.
A traditional area of concern in the design of rotary drill bits of this type involves the configuration at the center of rotation of the bit cutting face where the linear speed of the cutter elements is relatively slow.
It has been heretofore proposed to provide a drill bit with a concave recess at the center of the cutting face. When cutting through a hard substance, a core of the substance is formed within the recess. The core is gradually broken up by cutter elements disposed within the recess and/or by an inclined surface disposed at an inner end of the recess. The inclined surface also deflects the cuttings laterally through a discharge passage in the bit body. A drill bit of that type has utility in the cutting of both hard and soft substances. In the latter, the lateral discharge passage promotes the discharge of a high volume of cuttings which might otherwise clog-up the recess; thus, cutting may proceed at a faster rate. Exemplary of drill bits of that type are the bits disclosed in Grady U.S. Pat. No. 2,931,630 issued April 5, 1960; Abplanalp U.S. Pat. No. 3,727,704 issued April 17, 1973; and Rowley U.S. Pat. No. 4,234,048 issued Nov. 18, 1980.
Drill bits of the above-described type are subject to certain shortcomings, however, as determined by the present inventor. For example, as the core is engaged and broken up by the inclined surface at the inner end of the recess, an unbalanced force pattern is established on the drill bit. That is, the force generated by the contact between core and the inclined surface includes a radial component which tends to displace the drill bit from its intended travel path. As a result, it becomes difficult to cut in a predetermined straight path.
Furthermore, in cases where the discharge passage in the bit body extends longitudinally all the way to the forward end of the bit, there may be a tendency for drilling fluid to bypass some of the cutting elements mounted on the bit. That is, such a discharge passage forms a convenient travel path for drilling fluid to travel directly from the nozzle to the annulus without contacting the cutter bits to flush and cool same.
It is, therefore, an object of the present invention to minimize or obviate problems of the above-described sort.
Another object is to provide a drill bit which has utility in hard and soft substances and which promotes drilling in a straight direction.
A further object is to provide such a drill bit which minimizes tendencies for drilling fluid to bypass the cutter elements.
An additional object is to provide a centrally recessed drill bit which fractures a core without generating appreciable unbalanced forces on the bit.
A further object is to provide such a drill with a dam that deflects drilling fluid toward cutter elements which otherwise would be bypassed.